FIELD OF THE INVENTION
The invention relates to an electronic voice circuit configuration which can be connected through a first and a second line terminal to a telephone line and which has a calling part, a transmitting device, a receiver device, an equipment impedance device and a control device.
The coupling of telephone accessories, which operate at higher and higher frequencies, to the analog line interface increases the risk of wanted/interference radiation being coupled into audio amplification paths and leads to interference with operation. Demodulation products caused in that way can no longer be kept away from the baseband of the analog transmission channel.
For complexity and cost reasons, all known electronic voice circuits are asymmetric with respect to the reference ground which is defined between floating potentials of a,b wires themselves. In general, the reference ground potential, to which all coupled inputs are short-circuited, is always very close to the negative line wire. The desired operating resistance of the equipment, across which the operating voltage is dropped, is located towards the positive line wire. Interference that is coupled-in is short-circuited to the reference ground of the voltage supply. That results in an interference loop, which makes itself evident as a hum voltage in the line network.
That asymmetry increases the difficulties of structural layout to avoid interference coupling. If transformers are used, such as those which are utilized in classic analog switching technology, those problems are very largely overcome since the supply and receiver ground potential is always at half the supply voltage, and a new, independent ground reference can be selected through a magnetic coupling to the secondary of the transformer circuit. That procedure is uneconomical for terminal equipment for telephones, for cost reasons.